1. Field of the Invention
This invention relates to an illumination arrangement for supplying a uniform illuminating light of high brightness, and in particular to an arrangement for rendering the coherency of illumination suitable.
2. Description of the Prior Art
In an apparatus for effecting the photolithography used for the manufacture of semiconductors such as VLSI, it is necessary to effect uniform illumination of high brightness, and generally a super-high pressure mercury lamp is used for such illumination and an elliptical reflecting mirror is often used for condensing the illumination. Recently, however, a higher degree of integration of VLSI has become desired and not only the uniformity of the illumination but also illumination of higher brightness has become necessary. A laser is known as a light source of high brightness, but the laser has strong coherency and therefore forms interference fringes on the illuminated object surface and thus has been disadvantageous for uniform illumination.
In an imaging optical system, it is necessary to make the coherency of illumination into a suitable value in order to obtain a necessary resolving power, and it is known to control the .sigma. value (the ratio of the numerical aperture of the illuminating system to the numerical aperture of the objective lens) for this purpose. Where a coherent light source such as laser light is an illuminating light source, the light source is condensed into a spot and it is scanned at the pupil position of the imaging optical system by a suitable size, whereby the required coherency can be obtained. However, where the area to be scanned is great, the optical system for scanning becomes bulky and there is also a disadvantage that the time required for the scanning is long.